X-Ray Tomography Apparatus and Artifact Reducing Method

1. An X-ray computed tomography (CT) apparatus comprising: a scan device for exposing X rays to a subject while at least one of a gantry and a table is being moved along a body-axis direction of the subject, to create projection data of the subject; a CT value change determining device for determining an amount of change in CT value in the body-axis direction between a plurality of tomographic images obtained by backprojecting the projection data, with respect to each of pixel areas contained in the tomographic images; an image processing condition selecting device for estimating a range of an amount of change in CT value caused by an artifact, and for selecting an image processing condition for performing an image process for reducing artifacts based on whether the determined amount of change in CT value falls within the estimated range of the amount of change in CT value caused by an artifact, wherein the estimated range includes the amount of change in CT value smaller than that estimated to be caused by a structure of the subject and larger than that estimated to be caused by noise within the tomographic image; and an artifact reducing device for performing an image process using the image processing condition selected by the image processing condition selecting device.

2. The X-ray computed tomography apparatus according to claim 1 , further including: an artifact ratio calculating device for calculating a ratio of a number of pixel areas in which the amount of change in CT value is within a predetermined range, to a total number of pixel areas in each of the tomographic images; and a determining device for determining that when the ratio is larger than a threshold value, an image process for reducing the artifacts is effected on the pixel area.

3. The X-ray computed tomography apparatus according to claim 1 , wherein the image process of the artifact reducing device includes a process for multiplying, a plurality of pixel areas in the body-axis direction of the tomographic images by weighting factors based on the image processing condition, and adding the results of the multiplication.

4. The X-ray computed tomography apparatus according to claim 3 , wherein the image processing condition selecting device sets the weighting factor to be larger than 0 when the amount of change in CT value is within a predetermined range.

5. The X-ray computed tomography apparatus according to claim 3 , wherein the image processing condition selecting device changes the weighting factor according to the number of the pixel areas in the body-axis direction.

6. The X-ray computed tomography apparatus according to claim 3 , wherein the image processing condition selecting device changes the weighting factor according to the amount of change in CT value.

7. The X-ray computed tomography apparatus according to claim 1 , wherein the CT value change determining device determines an index value based on the amount of change in CT value, the index value corresponding to a set of weighting factors.

8. The X-ray computed tomography apparatus according to claim 7 , wherein plural pixel areas in the body-axis direction are multiplied by the weighting factors in relation to the pixel areas and the results of the multiplication are added together.

9. The X-ray computed tomography apparatus according to claim 2 , wherein the predetermined range expressed in Hounsfield units (HU) is from 3 HU to 300 HU.

10. The X-ray computed tomography apparatus according to claim 1 , wherein each of the pixel areas comprises one pixel or a plurality of pixels.

11. A method of reducing artifacts of X-ray computed tomographic (CT) images comprising: determining an amount of change in CT value in a body-axis direction between a plurality of tomographic images obtained by backprojecting projection data of a subject obtained by exposing X rays to the subject while at least one of a gantry and a table is being moved along the body-axis direction of the subject, with respect to each of pixel areas contained in the tomographic images; estimating a range of an amount of change in CT value caused by an artifact, wherein the estimated range includes the amount of change in CT value smaller than that estimated to be caused by a structure of the subject and larger than that estimated to be caused by noise within the tomographic image; selecting an image processing condition for performing an image process to reduce artifacts based on whether the determined amount of change in CT value falls within the estimated range of the amount of change in CT value caused by an artifact; and performing an image process using the selected image processing condition.

12. The artifact reducing method according to claim 11 , further comprising: calculating a ratio of a number of pixel areas in which the amount of change in CT value is within a predetermined range, to a total number of pixel areas in each of the tomographic images; and determining that when the ratio is larger than a threshold value, the image process for reducing the artifacts is effected on each of the pixel areas.

13. The artifact reducing method according to claim 11 , wherein the image process for reducing the artifacts includes a process for multiplying a plurality of pixel areas in the body-axis direction of the tomographic images by weighting factors based on the image processing condition, and adding the results of the multiplication.

14. The artifact reducing method according to claim 13 , wherein the image processing condition selecting step sets the weighting factor to be larger than 0 when the amount of change in CT value is within a predetermined range.

15. The artifact reducing method according to claim 13 , wherein the weighting factor is changed according to the number of the pixel areas in the body-axis direction.

16. The artifact reducing method according to claim 13 , wherein the weighting factor is changed according to the amount of change in CT value.

17. The artifact reducing method according to claim 11 , wherein an index value is determined based on the determined amount of change in CT value, the index value corresponding to a set of weighting factors.

18. The artifact reducing method according to claim 13 , wherein the weighting factor is changed according to each region of the subject.

19. The artifact reducing method according to claim 11 , wherein when the amount of change in CT value expressed in Hounsfield units (HU) ranges from 3 HU to 300 HU, an artifact is reduced with respect to each of the pixel areas.

20. The artifact reducing method according to claim 11 , wherein when the amount of change in CT value expressed in Hounsfield units (HU) ranges from 0 HU to 300 HU, an artifact is reduced with respect to each of the pixel areas and noise is reduced with respect to the pixel area.